Ceramics are utilized in many fields, because they are superior in electric and magnetic characteristics as well as mechanical properties such as heat resistance and abrasion resistance. In particular, ceramic sheets composed mainly of zirconia has excellent oxygen ion conductivity, heat resistance, corrosion resistance, rigidity, chemical resistance, and the like, so that they are widely utilized as a solid electrolyte film for sensor components such as oxygen sensors or moisture sensors, and as a solid electrolyte film for fuel cells.
The solid electrolyte films for fuel cells are required to have an enlarged contact area with electrodes for increasing an effective area of the cell reaction to improve electric power generation performance. In addition, for preventing electrode layers from peeling off the solid electrolyte films, desired are those having suitable surface unevenness on their surfaces. For these reasons, when solid electrolyte films are prepared, there have been studied surface roughing methods, for example, by blast processing of the surfaces, or by application of a coarse-grained zirconia paste and then calcining it to give a texture layer.
The present inventors have also studied surface roughness of zirconia sheets which can improve adhesion to electrode layers. As a part of the study, they have found that zirconia sheets having a maximum roughness (Ry) of 0.3 to 3 μm and an arithmetical mean roughness value (Ra) of 0.02 to 0.3 μm, as defined by JIS B-0601, exhibit excellent adhesion at the interface between the sheet surfaces and the electrode printed layers, thereby hardly causing peeling, and as a means of achieving such surface roughness, they have developed and proposed a method of controlling a particle size distribution in a slurry state (Japanese Patent Application Publication No. 2000-281438).
However, no relationship has been found between the surface roughness of electrolyte surfaces and the electric power generation performance. In other words, all that the method disclosed in this publication takes into consideration is adhesion between the electrolyte surfaces and the electrode layers against time-lapse peeling of the electrode layers from the electrolyte surfaces during the electrode reaction. The method completely fails to take into consideration the relationship between the surface roughness of electrolyte sheets and the electric power generation characteristics, and the publication contains no description of an electrolyte sheet having both acceptable adhesion to electrode printed layers and acceptable electric power generation characteristics.
Japanese Patent Application Publication No. 2000-351669 discloses a LaGaO3 oxide sintered material with a specific portion recessed from a mean line of a roughness curve as defined by JIS B-0601. However, this publication does not disclose surface roughness and electric power generation performance and fails to describe an electrolyte sheet having both acceptable adhesion of electrode printed layers and acceptable electric power generation characteristics, because the invention of this publication has been developed as a technique of obtaining a compact sintered material with high density.
As a means of improving the amount of electric power generation per unit area, there has been proposed a method comprising screen printing coarse particles composed of 8 mol % of yttrium oxide-stabilized zirconia (hereinafter referred to as “8YSZ”) and 10 mol % of scandium oxide-stabilized zirconia (hereinafter referred to as “10ScSZ”) on a solid electrolyte green sheet to allow the coarse particles to be adhered to the surface of the solid electrolyte sheet, followed by calcining, thereby increasing a contact area with electrodes and increasing an electrode reaction area with electrode layers (Abstracts of the Fourth European SOFC Forum, p. 697, 2000, and Abstracts of the Seventh Solid Oxide Fuel Cell International Symposium (SOFC VII), p. 349, 2001).
However, these reports also fail to make mention of the relationship between the surface roughness on the surface of an electrolyte and the electric power generation performance.
Further, Japanese Patent Application Publication No. 2002-42831 discloses a release film suitable for formation of a green sheet which contributes achievement of high-power fuel cells. However, this release film can hardly be said to be satisfactory because it intends to enlarge the surface area of one surface of a solid electrolyte and the increase rate of current density is about 4% at most. Even with the use of the technique disclosed in this publication, there cannot be obtained an electrolyte sheet having both acceptable adhesion of electrode printed layers and acceptable electric power generation characteristics.
As a general surface roughening method, a blast processing method has been known, and for example, as a means of surface roughening semiconductor wafer chucks or surface roughening the underside surface of an SAW filter substrate, a blast processing method has been known. However, when the surface of a very thin solid electrolyte film having a film thickness of about 0.5 mm or smaller, particularly 200 μm or smaller, is subjected to blast processing, there arises a problem that warp, undulation, and the like may occur on the solid electrolyte film due to a stress exerted by the blast processing. In other words, in the current technical level, it is difficult to physically roughen the surface of a solid electrolyte sheet having a small film thickness.
In addition, the method in which the surface of a solid electrolyte film is subjected to blast processing, and the pretreatment method in which a coarse-grained zirconia paste is applied to the surface of a solid electrolyte, followed by calcining to thereby provide a texture layer, are not suitable for mass production and may cause an increase in cell production cost. Some improvement is therefore required.
There may exist some demand for electrolyte sheets having both acceptable adhesion of electrode printed layers and acceptable electric power generation characteristics. However, there is known no disclosure of a specific method for obtaining an electrolyte sheet meeting such demand.
The present invention has been made taking note of the above circumstance, and it is an object of the present invention to aim at an electrolyte sheet for solid oxide fuel cells, which is a compact sintered sheet having a bulk density of 97% or higher relative to the theoretical density and having a gas permeability of substantially zero, and which is subjected to electrode printing on its both surfaces, and provide en electrolyte sheet, which can improve electric power generation characteristics, which have stably excellent thermal and mechanical characteristics, and in addition, which enables strong bonding of electrode printing on its surfaces with high adhesion, and which stably exhibits excellent performance without causing a deterioration of electric power generation characteristics by peeling or the like. It is another object of the present invention to establish a technique enabling efficient production of such a high-performance electrolyte sheet by a method with high industrial mass productivity.